I have kept sailfin, hippo, tomini, yellow, clown, purple and achilles tangs and none have ever bothered a clam.
They will pick at it's shell to get algae, but never had one take an active interest in picking at the mantle.
Naturally, the "oddballs" are out there which act out of character, but I think the odds are with you having luck with a tang.
JMO